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Re: Caused by unknown alignment, was: Re: On the x86_64, does one have to zero a vector register before filling it completely ?
On Sun, Nov 29, 2009 at 7:12 PM, Toon Moene <toon@moene.org> wrote:
> Toon Moene wrote:
>
>> This is where IPA could help. ?I created the following main program:
>>
>> ? ? ?real a(10), b(10), c(10)
>> ? ? ?a = 0.
>> ? ? ?b = 1.
>> ? ? ?print '(3(1x,z16))', loc(a), loc(b), loc(c)
>> ? ? ?call sum(a, b, c, 10)
>> ? ? ?print *, c(5)
>> ? ? ?end
>
>> So the alignment of a, b and c is known and is correct for vectorization -
>> still the loop in the subroutine looks like this (objdump -S a.out):
>
> Inlining the "sum.f" subroutine by hand:
>
> ? ? ?integer i
> ? ? ?real a(10), b(10), c(10)
> ? ? ?a = 0.
> ? ? ?b = 1.
> ? ? ?print '(3(1x,z16))', loc(a), loc(b), loc(c)
> ? ? ?do i = 1, 10
> ? ? ? ? c(i) = a(i) + b(i)
> ? ? ?enddo
> ? ? ?print *, c(5)
> ? ? ?end
>
> *does* lead to better code:
>
> ? ? ? ?movaps ?1056(%rsp), %xmm0
> ? ? ? ?movq ? ?%rbp, %rdi
> ? ? ? ?addps ? 1008(%rsp), %xmm0
> ? ? ? ?movq ? ?$.LC2, 488(%rsp)
> ? ? ? ?movaps ?%xmm0, 960(%rsp)
> ? ? ? ?movl ? ?$9, 496(%rsp)
> ? ? ? ?movaps ?1072(%rsp), %xmm0
> ? ? ? ?movl ? ?$128, 480(%rsp)
> ? ? ? ?addps ? 1024(%rsp), %xmm0
> ? ? ? ?movl ? ?$6, 484(%rsp)
> ? ? ? ?movaps ?%xmm0, 976(%rsp)
> ? ? ? ?movss ? 1088(%rsp), %xmm0
> ? ? ? ?addss ? 1040(%rsp), %xmm0
> ? ? ? ?movss ? %xmm0, 992(%rsp)
> ? ? ? ?movss ? 1092(%rsp), %xmm0
> ? ? ? ?addss ? 1044(%rsp), %xmm0
> ? ? ? ?movss ? %xmm0, 996(%rsp)
>
> i.e., a completely unrolled and (SLP) vectorized code.
>
> So the potential is there - what we just need is an Alignment Propagation
> Pass (analogous to the Constant and the Range Propagation pass).
Such a thing already existed a few years ago (IIRC Haifa had something
that Dan picked up and passed on to me). But it never brought any
benefits. I don't have the pass anymore, but perhaps Dan still has a
copy of it somewhere.
Ciao!
Steven